1. Field of the Invention
The present invention relates to an electroluminescence device and display unit. More specifically, the present invention relates to an electroluminescence device having a distributed type of structure in which electroluminescent particles, each having a fluorescent material, are distributed in a luminescent layer, and a display unit using such an electroluminescence device.
2. Description of the Related Art
The electroluminescence device is a prospective device for use as a self-luminescent surface light source in a new display unit and similar devices that require no additional light source. Two types of electroluminescence devices are known, one of which is referred to as “particle distributed type” and other is referred to as “thin film type”. The particle distributed type has a luminescent layer having fluorescent particles freely distributed in a dielectric binder. The thin film type has a luminescent layer formed in a thin film of a fluorescent material through a vacuum deposition process or the like. Of the two types, the particle distributed electroluminescence device is superior to the thin film electroluminescence device, in that it is manufactured through a simple manufacturing process that involves no vacuum deposition, which allows a low manufacturing cost, and manufacture of a large-sized device and even a flexible device having flexibility.
The basic structure of the particle distributed electroluminescence device as described, for example, in a non-patent document entitled “Electroluminescent Display” by Toshio Inoguchi, first edition, Sangyo-Tosho, Ltd., Jul. 25, 1991 is identical to that of a device 90 shown in FIG. 7. It has a transparent substrate 92 made of a glass or polyethylene terephthalate (PET) sheet having a transparent electrode 94 thereon formed in a layer of a transparent conductive material. It also has a luminescent layer 96, an insulation layer 98, and a back plate electrode 100 stacked on the electrode layer 94. In addition, additional layers, such as a surface protection layer, may also be provided. The luminescent layer 96 has fluorescent particles 102 freely distributed in a dielectric binder 104. When an AC voltage is applied between the transparent electrode 94 and back plate electrode 100, the fluorescent particles embedded in the luminescent layer 96 produce electroluminescent emissions, and the electroluminescent emissions produced in this manner exit through the transparent electrode 94 and transparent substrate 92. The insulation layer 98 is provided for blocking the current path so that a high voltage is applied stably to each of the fluorescent particles 102. However, the insulation layer 98 may not be required if the fluorescent particles 102 are distributed in complete freedom and a current path within the luminescent layer 96 is blocked off.
The luminescent color produced by the device is dependent on the type of the fluorescent particles used. Fluorescent particles that produce different luminescent colors may be mixed together to obtain a desired color, such as white. Dyestuffs and pigments may also be used to adjust the luminescent color. Further, the construction of so called “dual pattern system” or “triple pattern system” in which luminescent sections having particles that produce different luminescent colors are arranged two-dimensionally in parallel may also be employed. Still further, a plurality of structures identical to that shown in FIG. 7, each using a different type of fluorescent particles that produce a different luminescent color may be stacked in layers with all the intermediate electrodes made transparent. Thus, the electroluminescence device may provide multicolor representation so that it may be applied to color displays and similar devices.
As for the fluorescent particles for the particle distributed electroluminescence device, particles, such as ZnS:Cu, Cl, in which activators are added to the base material, are used in the typical known devices. The elements added as activators act as donors and accepters, and the device may produce luminescent emissions through recombination of these donors and accepters. For example, in the case of ZnS:Cu, Cl, Cl acts as donors and Cu acts as accepters. In the mean time, U.S. Patent Application Publication No. 20040119400 proposes an electroluminescence device which is identical in construction to that shown in FIG. 7, but employs fluorescent materials such as ZnS:Mn and others in particle form, which have conventionally been used for the luminescent layer of the thin film electroluminescence device as freely distributed fluorescent particles within the luminescent layer. In these devices, electrons are injected into the fluorescent sections from the interface between the fluorescent sections and dielectric sections surrounding thereof or from the trap and the like within the fluorescent sections. It is presumed that the similar mechanism to that of the conventional thin film device for producing luminescent emissions is realized through the collision excitation of the luminescence centers within the fluorescent sections by the hot electrons produced by the acceleration of these electrons. The basic structure of the device is identical to that of the conventional particle distributed electroluminescence device, so that the manufacturing cost may be kept low.
Here, as described above, the particle distributed electroluminescence device is superior to the thin film electroluminescence device in manufacturing cost, etc., but is inferior in brightness and the like. For this reason, various efforts have been made for realizing a particle distributed electroluminescence device having greater brightness and luminous efficiency, while maintaining advantages, such as a low manufacturing cost. As one of the examples of such efforts, U.S. Patent Application Publication No. 20040119400 proposes a particle distributed electroluminescence device that uses fluorescent particles comprising a dielectric core and a fluorescent coating layer, and also particles further provided with a dielectric layer covering the fluorescent coating layer so that the electric field is applied to the fluorescent coating layer effectively by the effect of the dielectric core.
However, known particle distributed electroluminescence devices have drawbacks, such as limited freedom in controlling the luminescent color for realizing a desired color, luminescent color change over time due to deterioration of the device, heavy circuit load when applied to multicolor representation, and the like.
That is, in controlling the luminescent color of the particle distribute electroluminescence device that uses the mixture of fluorescent particles that produce different luminescent colors for obtaining a desired color such as white, if the drive voltage is changed to adjust the brightness, the color is also changed due to the fact that, in general, each type of the particles has different voltage-brightness characteristics. This has caused a problem that the desired color is not obtained under various driving conditions and over a wide rage of brightness. Further, when the luminescent color is adjusted using dyestuffs or pigments, the amount of dyestuffs or pigments is determined according to a particular level of brightness, so that the color is changed significantly here also when the brightness is adjusted. Again, this has caused the problem that the desired color is not obtained under various driving conditions and over a wide rage of brightness.
For the change in the luminescent color over time due to deterioration of the device, when the mixture of fluorescent particles that produce different luminescent colors are used to obtain a desired color, the color changes over time due to the difference in deterioration pattern of the emission lifetime among the particles. Devices constructed with luminescent sections having particles that produce different luminescent colors arranged in parallel or stacked in layers has the same problem due to the difference in deterioration pattern of the emission lifetime among the particles used in each of the luminescent sections. Further, when the luminescent color is adjusted using dyestuffs or pigments, the difference in deterioration characteristics of the fluorescent materials due to electrical factors and the difference in deterioration characteristics of the dyestuffs or pigments due to optical factors such as ultraviolet rays cause here also the change in the luminescent color over time.
For the heavy circuit load when applied to multicolor representation, the difference in voltage-brightness characteristics among the particles used in each of the luminescent sections for each color causes the difference in voltage level applied to respective luminescent sections, resulting in increased circuit load.
Further, when fluorescent particles having sharp response in the voltage-brightness characteristics, or those having a dielectric core with a fluorescent coating layer are used in the particle distributed luminescence device in order to obtain high brightness and luminous efficiency, a small adjustment of the voltage causes a significant change in the brightness of the luminescence, thereby causing the problem that the representation of a delicate gradation sequence is difficult to obtain.